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Article

The Triterpenoids from Munronia pinnata and Their Anti-Proliferative Effects

by
Xuerong Yang
1,†,
Peiyuan Liu
1,3,†,
Yulu Wei
1,
Jingru Song
1,
Xiaojie Yan
1,
Xiaohua Jiang
1,
Jianxing Li
4,
Xiangqin Li
4,
Dianpeng Li
1,2,* and
Fenglai Lu
1,*
1
Guangxi Key Laboratory of Plant Functional Phytochemicals and Sustainable Utilization, Guangxi Institute of Botany, Guangxi Zhuang Autonomous Region and Chinese Academy of Sciences, No. 85 Yanshan Road, Guilin 541006, China
2
Engineering Research Center of Innovative Traditional Chinese, Zhuang and Yao Materia Medica, Ministry of Education, Guangxi University of Chinese Medicine, No. 13 Wuhe Road, Nanning 530200, China
3
School of Pharmacy, Guilin Medical University, No. 1 Zhiyuan Road, Guilin 541199, China
4
Guangxi Key Laboratory of Plant Conservation and Restoration Ecology in Karst Terrain, Guangxi Institute of Botany, Guangxi Zhuang Autonomous Region and Chinese Academy of Sciences, No. 85 Yanshan Road, Guilin 541006, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Molecules 2023, 28(19), 6839; https://doi.org/10.3390/molecules28196839
Submission received: 24 July 2023 / Revised: 14 September 2023 / Accepted: 20 September 2023 / Published: 28 September 2023
(This article belongs to the Special Issue Recent Advances in the Research of Natural Products: Isolation, Identification, Physicochemical Properties and Bioactivities)
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Abstract

:
Six new tirucallane-type triterpenoids, named munropenes A–F (1–6), were extracted from the whole plants of Munronia pinnata using a water extraction method. Their chemical structures were determined based on detailed spectroscopic data. The relative configurations of the acyclic structures at C-17 of munropenes A–F (1–6) were established using carbon–proton spin-coupling constants (2,3JC,H) and inter-proton spin-coupling constants (3JH,H). Furthermore, the absolute configurations of munropenes A–F (1–6) were determined through high-performance liquid chromatography (HPLC), single-crystal X-ray diffraction, and electronic circular dichroism (ECD) analyses. The antiproliferative effects of munropenes A–F were evaluated in five tumor cell lines: HCT116, A549, HepG2, MCF7, and MDAMB. Munropenes A, B, D, and F (1, 2, 4, and 6) inhibited proliferation in the HCT116 cell line with IC50 values of 40.90, 19.13, 17.66, and 32.62 µM, respectively.

1. Introduction

Munronia pinnata (Wall.) W. Theob., also called M. henryi Harms, belongs to the family Meliaceae [1,2]. It is a low subshrub that is naturally distributed in several countries, including India, China, the Philippines, and others [3]. In traditional Chinese medicine, Munronia pinnata is recognized for its efficacy in treating tuberculosis, cough, stomach pain, and sores [1,2,3,4].
Numerous structurally diverse compounds have been extracted from this plant, including limonoids, triterpenoids, flavonoids, lignans, sterols, sesquiterpenoids, and diterpenoids [3,5,6]. These compounds exhibit a wide range of bioactivities, such as anti-inflammatory, antiproliferative, anti-tobacco mosaic virus, and insect antifeedant activities [3,7,8,9], and have various roles in preserving food, flavoring, and treating various illnesses. In the early stages of our research, phytochemical study on the aerial parts of Munronia pinnata was isolated six novel limonoids [10]. In this study, six triterpenoids, named munropenes A–F (1–6) (Figure 1) were obtained from the whole plants of Munronia pinnata using a water extraction method, and their antiproliferative activities against several tumor cell lines, including HCT116, A549, HepG2, MCF7, and MDAMB acquired from the cell bank of Chinese Academy of Sciences (Shanghai, China) were also carried out.

2. Results and Discussion

In this investigation, compounds 1 and 2 were obtained as colorless amorphous solids with optical activity: {[α]D20.1 + 4.98 (c 0.10, MeOH) for 1; [α]D20.1 − 5.00 (c 0.10, MeOH) for 2}. The results of HRESIMS suggest that compounds 1 and 2 have the same molecular formula—C32H50O12 {m/z 625.3354 ([M − H]−, Δ + 12.4 mmu) for 1 and m/z 625.3136 ([M − H]−, Δ − 9.4 mmu) for 2}—indicating that both compounds are isomers with eight degrees of unsaturation. The IR spectrum showed the presence of carbonyl functionalities at 1748 cm−1 and 1689 cm−1 for 1 and 2. The 1H NMR spectrum revealed the existence of one trisubstituted olefin, eight sp3 methines, seven sp3 methylenes, and six singlet methyls (including one acetyl methyl). The 13C NMR spectrum displayed 32 signals, including 3 ester carbonyls, 2 olefinic, 2 oxygenated tertiary, and 3 quaternary carbon signals (Table 1). These data indicate a tetracyclic triterpene structure for compounds 1 and 2, with the primary difference being the configuration at C-7.
The tetracyclic ring moiety includes an α,β-unsaturated-ε-caprolactone ring (C-1–C-5, C-10) with a formyl group and a methoxy group at C-4, as well as three methyl groups at C-8, C-10, and C-13. The 1H-1H COSY cross-peaks were observed among H-1/H2-2, H-5/H2-6/H-7, H-9-H2-11/H2-12, and H-15/H2-16/H-17. Additionally, HMBC correlations were found between H-1 and C-3; H2-28 and C-4, C-5, and C-29; H3-30 and C-7, C-8, C-9, and C-14; H3-19 and C-1, C-5, C-9, and C-10; H3-18 and C-12, C-13, C-14, and C-17; and H-15 and C-14 (Figure 2). From the IR spectrum and the degree of unsaturation of compound 1, it was determined that an α,β-unsaturated-ε-caprolactone ring is present. Furthermore, the presence of an acetoxy group at C-11 was elucidated by the heteronuclear multiple bond correlation (HMBC) between H-1 and the acetoxy carbonyl carbon. The presence of a 1,4,5,6-tetrahydroxy-6-methyl-heptanol moiety at C-17 was suggested based on the 1H-1H COSY cross-peaks of H-17/H-20 and H2-21/H-20/H2-22/H-23/H-24, as well as the HMBC correlations between H3-26 and C-24, C-25, and C-26, and H-21 with C-20 and C-22. In short, the planar structure of 1 was established as described.
The relative configuration of the tetracyclic ring moiety in compound 1 was primarily confirmed using the ROESY method (Figure 3). The rotating-frame nuclear Overhauser effect spectroscopy (ROESY) correlations of H3-19/H-1, H3-19/H-6b, H3-19/H3-30, H3-30/H-7, H3-30/H-17, H-16b/H-17, and H-7/H-6β suggested that these protons were cofacial and they were arbitrarily assigned as β-oriented. Consequently, the orientations of H-5, H-16a, H-9, H-18, and H2-29 were assigned as α-oriented based on the ROESY correlations between H2-28/H-5, H-5/H-6a, H-5/H-9, H-9/H3-18, and H3-18/H-16α. The relative configurations of C-20, C-23, and C-24 were determined based on J-based configuration analysis [11]. In addition to the 3JH,H values, 2,3JC,H values were detected using hetero half-filtered TOCSY (HETLOC) [12,13], phase-sensitive COSY (PS-COSY) [14], and phase-sensitive HMBC (PS-HMBC) [15,16] spectra of 1 in CD3OD. The relative magnitudes of coupling constants assigned from the 3JH,H and 2,3JC,H values indicated that each of C-17–C-20, C-20–C-22, C-22–C-23, and C-23–C-24 bonds adopted a single dominant conformer (Figure 4), which was further supported by the ROESY correlations (Figure 3). Thus, the relative configuration at C-20, C-23, and C-24 in compound 1 was assigned as R*, S*, and S*, respectively.
The confirmation was obtained through the utilization of single-crystal X-ray diffraction (Figure 5). Accordingly, the relative configuration of compound 2 was assigned as 1S*, 4S*, 5R*, 7R*, 8R*, 9R*, 10R*,13S*, 17S*, 20S*, 23R*, and 24R* based on the comparison of 1D NMR data (Table 1) and the 3JH,H and 2,3JC,H values of compound 2 with those of compound 1. This determination was further supported by ROESY correlations, such as H-9/H3-18 and H3-18/H-17, in compound 2 (Figure 3). The absolute configurations of compound 2 were assigned using the electronic circular dichroism (ECD) spectrum since obtaining a crystal for single-crystal X-ray diffraction data was not possible. The TDDFT {CAM-B3LYP/6-31G + (d)} calculation of a possible enantiomer (1S*, 4S*, 5R*, 7R*, 8R*, 9R*, 10R*,13S*, 17S*, 20S*, 23R*, 24R*) of compound 2 yielded a calculated ECD spectrum that matched the experimental spectrum of compound 2 (Figure 6), confirming the 1S*, 4S*, 5R*, 7R*, 8R*, 9R*, 10R*,13S*, 17S*, 20S*, 23R*, and 24R* configurations of compound 2. Thus, the chemical structures of compounds 1 and 2 were established as shown in Figure 1.
Munropene C (compound 3) was obtained as an optically active, colorless amorphous solid. The specific rotation [α]D20.0 = −50.51 (c 0.10, MeOH) indicated its optical activity. From the HRESIMS, a sodiated molecular ion at m/z 643.3221 ([M − H]−, Δ−11.4 mmu) was observed, revealing the molecular formula of compound 3 to be C32H52O13, suggesting the presence of seven degrees of unsaturation. The 1D NMR spectra of compound 3 (Table 1) were similar to those of compound 1, except for signals related to ring A. By comparing the degrees of unsaturation and molecular formula of 3 with those of compound 1, it was concluded that compound 3 was a ring A-seco munropene A (1). This conclusion was further supported by the 1H NMR chemical shifts of H-1 and H-2 in compound 3 (Figure 2). Therefore, a possible biosynthetic pathway for munropene C (compound 3) was proposed, suggesting that it might be generated through the hydrolysis of munropene A (compound 1) in ring A (Figure S2).
The β orientation of H-5, H-7, H-17, H3-19, and H3-30 was assigned based on the ROESY correlations of H3-19/H-5, H3-19/H3-30, H3-30/H-7, H3-30/H-17, and H-7/H-5. The ROESY cross-peaks of H-9/H3-18 suggested the α orientations of H-9 and H3-18 (Figure S1). Thus, considering the similar biosynthetic pathway of compounds 1 and 3, the S* configuration was assigned to C-1 and C-4 of compound 3. The ROESY cross-peaks of H-1/H-5, H-1/H3-19, H2-29/H-5, and H2-29/H3-19 further supported this assignment (Figure S1). Consequently, the structure and relative configuration of compound 3 were established as shown.
The molecular formula of munropene D (compound 4) was determined to be C38H62O14 through HRESIMS analysis, showing a peak at m/z 741.4010 ([M − H]−, Δ−5.7 mmu). The 1D NMR spectra of compound 4 (Table 1) exhibited signals originating from a glucose group, indicating its structural similarity to that of compound 3, except for the modifications at the C-5 and C-7 positions. The attachment of a methylethylene moiety at C-5 was confirmed by the HMBC correlation of H3-29 with C-4, C-5, and C-28.
The sugar moiety was obtained through acid hydrolysis, followed by treatment with L-cysteine methyl ester and o-tolylisothiocyanate, resulting in a reaction mixture that produced a peak during HPLC analysis identical to that of the derivative of authentic D-glucose prepared using the same procedure [17]. Hence, the glucose moiety of compound 4 was determined to be D-glucose. The β-glycosidic linkage of the D-glucosyl moiety at C-7 was concluded based on the coupling constant value of the anomeric proton (H-1′, J = 7.8 Hz), as well as the HMBC correlation of H-1′ with C-7 (Figure 2).
The relative configurations of compound 4 were assigned as 1S*, 4S*, 5R*, 7R*, 8R*, 9R*, 10R*, 13S*, 17R*, 21R*, 23S*, and 24S* through a comparison of the NMR data of compound 4 with that of compound 1. ROESY cross-peaks observed in compound 4 between H3-19/H3-30, H3-30/H-7, H3-30/H-17, H-5/H-9, H3-18/H-9, H3-18/H-16a, and H-16b/H-17, which were also present in compound 1, supported this assignment.
Munropenes E (compound 5) and F (compound 6) were isolated as optically active colorless amorphous solids. Their optical rotations were determined as [α]D20.0 = −24.04 (c 0.10, MeOH) for compound 5 and [α]D20.0 = −6.80 (c 0.10, MeOH) for compound 6. HRESIMS analysis revealed the molecular formulas as C36H58O12 (m/z 727.3846 [M + HCOO]−, Δ−6.4 mmu for compound 5; m/z 727.3845 [M + HCOO]−, Δ−6.5 mmu for compound 6), suggesting the presence of eight degrees of unsaturation. The 1H NMR data (Table 2) displayed resonances corresponding to a trisubstituted olefin, a 1,2-disubstituted olefin, seven sp3 methines, seven sp3 methylenes, six singlet methyls, and a glucosyl moiety. The 13C NMR spectrum exhibited 36 signals, including 1 ketone carbonyl, 4 olefinic, 1 oxygenated tertiary, and 4 quaternary carbon signals (Table 2). These data indicate that compounds 5 and 6 are isomers of each other and closely related to compounds 1 and 2, except for changes occurring in the A ring and at C-7. The glucose moiety of compounds 5 and 6 was determined to be d-glucose through similar HPLC analyses as performed for compound 4. The comparison of the 1D NMR spectroscopic data of compounds 5 and 6 with those of compound 4 indicated that the β-glycosidic linkage of the d-glucosyl moiety was attached at C-7, which was confirmed by the HMBC correlation of H-1′ with C-7, as well as similar HPLC analyses as conducted for compound 4. The A rings of these two compounds were assigned as α, β-unsaturated hexane ketones with one methyl and one methanol group at C-4, elucidated by 1H-1H COSY cross-peaks of H-1/H-2 and the HMBC correlations of H3-29 with C-3, C-4, C-5, and C-28, as well as H-1 with C-3, C-5, and C-10. Additionally, the HMBC correlations of H3-19 with C-1, C-10, C-5, and C-9 supported this assignment and allowed for the connectivity between ring A and ring B.
The relative configurations of compounds 5 and 6 in the aglycone moieties were deduced to be similar to those of compounds 1 and 2, respectively, based on the resemblance of their 1D NMR data (Table 1 and Table 2) and ROESY correlations (Figure S2). The ECD spectra of compounds 5 and 6 indicated a similar Cotton effect at 237 nm and 203 nm. According to the octant rule [18], the positive Cotton effect observed in compounds 5 and 6, attributed to the exciton coupling of α, β-unsaturated hexane ketone, suggested the absolute configuration of 4S*, 5R*, and 10R* in compounds 5 and 6. The absolute configuration of compound 6 was confirmed by comparing the experimental ECD spectrum with the TDDFT calculated spectrum. The experimental ECD spectrum of compound 5 correlated well with the calculated spectrum of a possible enantiomer with the 4S*, 5R*, 7R*, 8R*, 9R*, 10R*, 13S*, 17R*, 20R*, 23S*, and 24S* configurations (Figure 7), confirming the assignment of the absolute configuration of compound 5 as mentioned above. Thus, the structures of compounds 5 and 6 were elucidated as shown in Figure 1.
As part of our ongoing search for potential natural product leads for therapeutic agents from M. pinnata, we evaluated the antiproliferative activity of munropenes A–F (compounds 1–6) against various human cancer cell lines including HCT116, A549, HepG2, MCF7, and MDAMB. Munropenes A, B, D, and F (compounds 1, 2, 4, and 6) were not cytotoxic (IC50 > 50 μM) to A549, HepG2, MCF7, and MDAMB cells. However, they exhibited moderate cytotoxicity against HCT116 cells, with IC50 values of 40.90, 19.13, 17.66, and 32.62 μM, respectively (Table 3). In contrast, munropenes C and E (compounds 3 and 5) did not exert any cytotoxicity against the tested cell lines (Table 3).

3. Materials and Methods

3.1. General Experimental Protocols

The Jasco P-1020 polarimeter was used to measure optical rotation. Infrared (IR) spectra were obtained using a Tensor 27 spectrometer and a Nicolet Fourier transform infrared spectrometer (Thermo Fisher, Waltham, MA, USA) with KBr pellets. Circular dichroism (CD) spectra were recorded using a J-810 CD spectrometer. MS spectra were measured using an LC/MS-IT-TOF mass spectrometer. Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker AVANCE III-HD 500 spectrometer with MeOH (δH 3.30 and δC 49.0) serving as the internal standard. The countercurrent chromatography (CCC) experiment was conducted using a TBE-300C machine (manufactured by Tauto Biotechnique, located in Shanghai, China). HPLC analysis was performed using an Agilent 1260 InfinityIILC system (Agilent Technologies, Santa Clara, CA, USA). The columns utilized were Agilent Poroshell 120 SB-C18 (4 mm, 4.6 mm × 150 mm, Agilent, Santa Clara, CA, USA), ChromCore 120-C18 (5 mm, 10 mm × 250 mm, NanoChrom, Suzhou, China), and Agilent ZORBAX SB-C18 (5 mm, 9.4 mm × 250 mm, Agilent, Santa Clara, CA, USA). Silica gel (200–300 mesh, Qingdao Marine Chemical Factory, Qingdao, China) and MCI gel (Mitsubishi Chemical Corporation, Tokyo, Japan) were used for column chromatography. Thin-layer chromatography (TLC) analyses were performed using preloaded silica gel 60 F254 plates from Merck Millipore in Germany. The spots were visualized by heating the silica gel plate, which was sprayed with a mixture of 10% H2SO4 and ethanol.

3.2. Plant Material

Botanical samples of Munronia pinnata (Wall.) W. Theob. were collected in July 2021 from Jingxi City, located in the Guangxi Zhuang Autonomous Region. The plant material was identified by one of the authors, X.-Q. Li. Voucher specimens have been preserved at the herbarium of the Center for Natural Products Chemistry Studies, Guangxi Institute of Botany, Guangxi Zhuang Autonomous Region, and Chinese Academy of Sciences (21-GX-001).

3.3. Extraction and Isolation

Munronia pinnata (25 kg), which had been air-dried and powdered, was extracted three times with 95% ethanol (250 L) under reflux conditions. The resulting mixture was then filtered to remove any insoluble components. The filtrate was concentrated under reduced pressure to obtain the extract. The extract was further extracted using petroleum ether and EtOAc, yielding a remaining water layer. To obtain fraction Fr 3 (34 g), the water layer was subjected to macroporous resin column chromatography with elution using 20%, 40%, and 80% ethanol. Additionally, the 80% fraction was subjected to gel column chromatography with methanol elution, yielding fraction Fr2 (15 g). Fr2 was separated by C18 column chromatography with a methanol gradient elution (MeOH-H2O, 30:70–50:50 gradient system) to obtain seven fractions (Fr2.1–Fr2.7). Fr2.1 (1.46 g) was further separated by HSCCC [CH2Cl2-MeOH-H2O (2:2:1, v/v/v)] followed by silica gel (CH2Cl2-MeOH, 4:1) to yield 2 (68 mg). Fr2.3 (1.27 g) was obtained by HSCCC [CH2Cl2-MeOH-H2O (2:2:1, v/v/v)]. Fr2.3.1–Fr2.3.3. Fr2.3.3 was subjected to prep-HPLC [MeCN-H2O-HCOOH (23:77:0.1, v/v/v)] to obtain compound 3 (26 mg, tR = 10.1 min). Fr2.4 (1.39 g) was isolated by HSCCC [CH2Cl2-MeOH-H2O (2:2:1, v/v/v)], and then compound 5 (10 mg, tR = 12.2 min) and compound 6 (15 mg, tR = 14.7 min) were obtained by prep-HPLC [MeCN-H2O-HCOOH (23:77:0.1, v/v/v)]. Fr2.7 (0.9 g) yielded compound 4 (184 mg) through HSCCC [CH2Cl2-MeOH-H2O (2:2:1, v/v/v)]. Fr.3 (30 g) was subjected to silica gel column chromatography (CH2Cl2-MeOH, 80:20–100:0 gradient system) resulting in twenty fractions (Fr3.1–Fr3.20). Fr3.3 (1.49 g) was purified by prep-HPLC [MeCN-H2O-HCOOH (18:82:0.1, v/v/v)] to obtain compound 1 (321 mg, tR = 21.3 min).

3.3.1. Munropene A (Compound 1)

Colorless amorphous solid; [a]D20.1 + 4.98 (c 0.10, 90% MeOH aq.); IR (KBr) nmax 3368 (-OH), 2968 (-CH), 1724 (-C=O), and 1636 (-C=C-) cm−1; UV (MeOH) λmax 209 (e = A/CL, 24,281) nm; 1H and 13C NMR (CD3OD/D2O, Table 1); HRESIMS m/z 625.3354 ([M − H]−, calcd for C32H49O12, 625.3230).

3.3.2. Munropene B (Compound 2)

Colorless amorphous solid; [a]D20.1 − 5.00 (c 0.10, 90% MeOH aq.); IR (KBr) nmax 3422 (-OH), 2930 (-CH), 1720 (-C=O), and 1620 (-C=C-) cm−1; UV (MeOH) λmax 197 (e = A/CL, 10,532) nm; ECD (MeOH) De (nm) + 6.0 (223); 1H and 13C NMR (CD3OD/D2O, Table 1); HRESIMS m/z 625.3136 ([M − H]−, calcd for C32H49O12, 625.3230).

3.3.3. Munropene C (Compound 3)

Colorless amorphous solid; [a]D20.0 − 50.51 (c 0.10, 90% MeOH aq.); IR (KBr) nmax 3422 (-OH), 2930 (-CH), 1731 (-C=O), and 1630 (-C=C-) cm−1; UV (MeOH) λmax 195 (e = A/CL, 13,859) nm; ECD (MeOH) De (nm) + 9.1 (227), +0.5 (197); 1H and 13C NMR (CD3OD/D2O, Table 1); HRESIMS m/z 643.3221 ([M − H]−, calcd for C32H51O13, 643.3335).

3.3.4. Munropene D (Compound 4)

Colorless amorphous solid; [a]D20.0 − 76.15 (c 0.10, 90% MeOH aq.); IR (KBr) nmax 3419 (-OH), 2927 (-CH), 1722 (-C=O), and 1636 (-C=C-) cm−1; UV (MeOH) λmax 195 (e = A/CL, 26,334) nm; ECD (MeOH) De (nm) + 1.2 (198), −6.3 (211); 1H and 13C NMR (CD3OD/D2O, Table 1); HRESIMS m/z 741.4010 ([M − H]−, calcd for C38H61O14, 741.4067).

3.3.5. Munropene E (Compound 5)

Colorless amorphous solid; [a]D20.0 − 24.04 (c 0.10, 90% MeOH aq.); IR (KBr) nmax 3421 (-OH), 2926 (-CH), 1722 (-C=O), and 1650 (-C=C-) cm−1; UV (MeOH) λmax 196 (e = A/CL, 12,066) nm; ECD (MeOH) De (nm) + 22.9 (235), −16.8 (204); 1H and 13C NMR (CD3OD/D2O, Table 1); HRESIMS m/z 727.3846 ([M + HCOO]−, calcd for C37H59O14, 727.3910).

3.3.6. Munropene F (Compound 6)

Colorless amorphous solid; [a]D20.0 − 6.80 (c 0.10, 90% MeOH aq.); IR (KBr) nmax 3412 (-OH), 2926 (-CH), 1722 (-C=O), and 1656 (-C=C-) cm−1; UV (MeOH) λmax 197 (e = A/CL, 13,228) nm; ECD (MeOH) De (nm) + 23.8 (235), −18.1 (203); 1H and 13C NMR (CD3OD/D2O, Table 1); HRESIMS m/z 727.3845 ([M + HCOO]−, calcd for C37H59O14, 727.3910).

3.4. Acid Hydrolysis and Sugar Analysis of Munropenes D–F (Compounds 4–6)

Compounds 4–6 (1.5 mg each) were subjected to hydrolysis using 2.0 M HCl (2.0 mL) for a duration of 2 h at a temperature of 90 °C. To establish neutral conditions, anion exchange resin (IRA 400) was added and subsequently removed through filtration. The resulting filtrate was then subjected to vacuum concentration and dried under vacuum conditions. The resultant residue was dissolved in pyridine (1.0 mL) supplemented with L-cysteine methyl ester hydrochloride (1.0 mg) and heated at 60 °C for 1 h. Subsequently, o-torylisothiocyanate (1.0 mg) was added to the mixture, which was then stirred at 60 °C for an additional hour. Reversed-phase HPLC was used to directly analyze the reaction mixture, and the retention times of reference compounds and carbohydrate derivatives were compared, which was performed under the following conditions: detection wavelength of 250 nm, mobile phase consisting of 25% acetonitrile–water with 0.1% formic acid, and utilizing an Agilent Poroshell 120 SB-C18 column (4 mm, 4.6 mm × 150 mm, Agilent, Santa Clara, CA, USA). The absolute conformation of the sugar moiety was ascertained through comparison with D-glucose (tR = 9.55 min).

3.5. Cytotoxicity Assay

The cytotoxicity of munropenes A–F (1–6) in A549, HepG2, HCT116, MCF7, and MDAMB was tested using the Cell Counting Kit-8 (CCK-8). A 100 μL cell suspension (2 × 105 cells/mL) was seeded into 96-well plates. Following incubation for 24 h, the cells were treated with various concentrations (5, 10, 20, 40, 80, or 160 µM) of each specific compound, while the control cells received an equal volume of DMSO. Subsequently, after an additional 24 h of culture, 10 μL CCK-8 was added and incubated for an additional 2 h. The absorbance value at 450 nm was detected using a microplate reader, enabling the calculation of the cell survival rate.

4. Conclusions

The phytochemical study on the whole plants of a Chinese traditional medicine plant Munronia pinnata (Meliaceae) led to the isolation of six new tirucallane-type triterpenoids, munropenes A–F (compounds 1–6). Tirucallane-type triterpenoids are known as major components of plants belonging to Meliaceae, but they had not been systematically studied in M. pinnata. In the present paper, munropenes A and B (1 and 2) were identified as tirucallane-type triterpenoids with an α,β-unsaturated-ε-caprolactone moiety in ring A, while munropenes C and D (compounds 3 and 4) were categorized as ring A seco-tirucallane-type triterpenoids. Additionally, munropenes D, E, and F (compounds 4, 5, and 6) were determined to be glycosides of tirucallane-type triterpenoids based on 1D and 2D-NMR, HR-ESI-MS, IR, single-crystal X-ray diffraction, ECD, and J-based configuration analyses. Munropenes A, B, D, and F (compounds 1, 2, 4, and 6) was moderately cytotoxic to the HCT116 cell line, but did not show any cytotoxicity in the A549, HepG2, MCF7, and MDAMB cell lines. Furthermore, munropenes C (compound 3) and E (compound 5) exerted no cytotoxicity against all tested cell lines, including HCT116, A549, HepG2, MCF7, and MDAMB cells.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules28196839/s1. Table S1. Partial 3JH,H and 2,3JC,H data for munropenes A–F (1–6). Figure S1. Possible biogenetic pathway of munropene C (3). Figure S2. Key NOESY correlations and relative configuration for munropenes C,D (3,4) (protons of methyl groups are omitted). Figure S3. Key NOESY correlations and relative configuration for munropenes E,F (5,6) (protons of methyl groups are omitted). Figure S4. Effects of different concentrations of munropenes A–F (compounds 1–6) on HCT116 cells viability. Figure S5. 1H NMR spectrum of munropene A (1) in CD3OD. Figure S6. 13C NMR spectrum of munropene A (1) in CD3OD. Figure S7. 1H–1H COSY spectrum of munropene A (1) in CD3OD. Figure S8. HSQC spectrum of munropene A (1) in CD3OD. Figure S9. HMBC spectrum of munropene A (1) in CD3OD. Figure S10. ROESY spectrum of munropene A (1) in CD3OD. Figure S11. IR spectrum of munropene A (1). Figure S12. 1H NMR spectrum of munropene B (2) in CD3OD. Figure S13. 13C NMR spectrum of munropene B (2) in CD3OD. Figure S14. 1H–1H COSY spectrum of munropene B (2) in CD3OD. Figure S15. HSQC spectrum of munropene B (2) in CD3OD. Figure S16. HMBC spectrum of munropene B (2) in CD3OD. Figure S17. ROESY spectrum of munropene B (2) in CD3OD. Figure S18. HETLOC spectrum of munropene B (2) in CD3OD. Figure S19. PS-HMBC spectrum of munropene B (2) in CD3OD. Figure S20. PS-COSY spectrum of munropene B (2) in CD3OD. Figure S21. IR spectrum of munropene B (2). Figure S22. CD spectrum of munropene B (2). Figure S23. 1H NMR spectrum of munropene C (3) in CD3OD. Figure S24. 13C NMR spectrum of munropene C (3) in CD3OD. Figure S25. 1H–1H COSY spectrum of munropene C (3) in CD3OD. Figure S26. HSQC spectrum of munropene C (3) in CD3OD. Figure S27. HMBC spectrum of munropene C (3) in CD3OD. Figure S28. ROESY spectrum of munropene C (3) in CD3OD. Figure S29. HETLOC spectrum of munropene C (3) in CD3OD. Figure S30. PS-HMBC spectrum of munropene C (3) in CD3OD. Figure S31. PS-COSY spectrum of munropene C (3) in CD3OD. Figure S32. IR spectrum of munropene C (3). Figure S32. CD spectrum of munropene C (3). Figure S34. 1H NMR spectrum of munropene D (4) in CD3OD. Figure S35. 13C NMR spectrum of munropene D (4) in CD3OD. Figure S36. 1H–1H COSY spectrum of munropene D (4) in CD3OD. Figure S37. HSQC spectrum of munropene D (4) in CD3OD. Figure S38. HMBC spectrum of munropene D (4) in CD3OD. Figure S39. ROESY spectrum of munropene D (4) in CD3OD. Figure S40. HETLOC spectrum of munropene D (4) in CD3OD. Figure S41. PS-HMBC spectrum of munropene D (4) in CD3OD. Figure S42. PS-COSY spectrum of munropene D (4) in CD3OD. Figure S43. IR spectrum of munropene D (4). Figure S44. CD spectrum of munropene D (4). Figure S45. 1H NMR spectrum of munropene E (5) in CD3OD. Figure S46. 13C NMR spectrum of munropene E (5) in CD3OD. Figure S47. 1H–1H COSY spectrum of munropene E (5) in CD3OD. Figure S48. HSQC spectrum of munropene E (5) in CD3OD. Figure S49. HMBC spectrum of munropene E (5) in CD3OD. Figure S50. ROESY spectrum of munropene E (5) in CD3OD. Figure S51. HETLOC spectrum of munropene E (5) in CD3OD. Figure S52. PS-HMBC spectrum of munropene E (5) in CD3OD. Figure S53. IR spectrum of munropene E (5). Figure S54. 1H NMR spectrum of munropene F (6) in CD3OD. Figure S55. 13C NMR spectrum of munropene F (6) in CD3OD. Figure S56. 1H–1H COSY spectrum of munropene F (6) in CD3OD. Figure S57. HSQC spectrum of munropene F (6) in CD3OD. Figure S58. HMBC spectrum of munropene F (6) in CD3OD. Figure S59. ROESY spectrum of munropene F (6) in CD3OD. Figure S60. HETLOC spectrum of munropene F (6) in CD3OD. Figure S61. PS-HMBC spectrum of munropene F (6) in CD3OD. Figure S62. IR spectrum of munropene F (6).

Author Contributions

Conceptualization, X.Y. (Xuerong Yang), F.L. and D.L.; Methodology, P.L. and Y.W.; Formal analysis, P.L., J.S., X.Y. (Xiaojie Yan) and X.J.; Investigation, X.Y. (Xuerong Yang) and P.L.; resources, J.L. and X.L.; Writing—original draft, X.Y. (Xuerong Yang) and P.L.; writing—review and editing, F.L.; supervision, D.L.; project administration, X.Y. (Xuerong Yang); funding acquisition, D.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (U20A2004), the Guangxi Science and Technology Base and Talent Project (Guike AA21196009), the Basic Research Fund of Guangxi Academy of Sciences (CQZ-C-1901), the Guilin Innovation Platform and Talent Plan (20210102-3), and the Key Area Research and Development Program of Guangdong Province (2020B1111110003).

Institutional Review Board Statement

The protocol was approved by the Research Ethics Committee of the Guangxi Institute of Botany, Guangxi Zhuang Autonomous Region and the Chinese Academy of Science (approval No: GXZW-20230225-2).

Informed Consent Statement

Not applicable.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors declare no conflict of interest.

Sample Availability

Samples of the compounds are available from the authors.

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Molecules 28 06839 g001
Figure 1. The chemical structures of munropenes A–F (1–6).
Figure 1. The chemical structures of munropenes A–F (1–6).
Molecules 28 06839 g001
Molecules 28 06839 g002
Figure 2. Selected 2D NMR correlations for munropenes A–F (1–6).
Figure 2. Selected 2D NMR correlations for munropenes A–F (1–6).
Molecules 28 06839 g002
Molecules 28 06839 g003
Figure 3. Key ROESY correlations and relative configuration for munropenes A, B (1, 2) (protons of methyl groups are omitted).
Figure 3. Key ROESY correlations and relative configuration for munropenes A, B (1, 2) (protons of methyl groups are omitted).
Molecules 28 06839 g003
Molecules 28 06839 g004
Figure 4. Rotation models for the (a) C-17–C-20, (b) C-20–C-22, (c) C-22–C-23, and (d) C-23–C-24 bonds of munropene A (1). “nd” means that the magnitude was not determined. Dashed arrows indicate ROESY correlations.
Figure 4. Rotation models for the (a) C-17–C-20, (b) C-20–C-22, (c) C-22–C-23, and (d) C-23–C-24 bonds of munropene A (1). “nd” means that the magnitude was not determined. Dashed arrows indicate ROESY correlations.
Molecules 28 06839 g004
Molecules 28 06839 g005
Figure 5. X-ray crystal structure of munropene A (1).
Figure 5. X-ray crystal structure of munropene A (1).
Molecules 28 06839 g005
Molecules 28 06839 g006
Figure 6. Experimental and calculated ECD spectra of munropene B (2).
Figure 6. Experimental and calculated ECD spectra of munropene B (2).
Molecules 28 06839 g006
Molecules 28 06839 g007
Figure 7. Experimental and calculated ECD spectra of 5.
Figure 7. Experimental and calculated ECD spectra of 5.
Molecules 28 06839 g007
Table 1. 1H and 13C NMR data for munropenes A–D (1–4) in CD3OD.
Table 1. 1H and 13C NMR data for munropenes A–D (1–4) in CD3OD.
Position1234
δH (J in Hz)δCδH (J in Hz)δCδH (J in Hz)δCδH (J in Hz)δC
14.66 (1H, d, 7.2)71.34.78 (1H, d, 7.5)73.36.21 (1H, d, 7.5)78.85.55 (1H, dd, 11.1, 1.6)79.1
23.31 (1H, d,16.0)/
2.77 (1H, m)		35.53.84 (1H, m)/
2.93 (1H, m)		37.12.41 (1H, m)/
2.99 (1H, m)		36.22.41 (1H, dd, 15.1, 11.0)/
3.08 (1H, m)		36.3
3-170.5-175.8-175.5-175.5
4-87.4-91.4-81.9-147.7
52.77 (1H, m)39.22.83 (1H, dd, 13.0, 2.6)40.32.37 (1H, dd, 13.0, 2.6)42.72.79 (1H, dd, 13.0, 3.4)43.9
62.15 (1H, m)/
1.87 (1H, m)		27.62.49 (1H, m)/
1.99 (1H, m)		28.82.29 (1H, m)/
1.68 (1H, m)		28.12.07 (1H, m)/
1.76 (1H, dt, 14.5, 3.4)		28.4
73.75 (1H, m)70.73.91 (1H, t, 3.1)733.85 (1H, t, 3.1)734.03 (1H, dd, 3.7, 1.5)78.4
8-42.8-44.5-44.6-44.3
92.40 (1H, m)33.32.52 (1H, m)34.82.29 (1H, m)35.32.30 (1H, m) 35.5
10-43.6-45.2-47.1-45.5
111.22 (1H, m)/
1.40 (1H, m)		16.41.47 (1H, m)/
1.52 (1H, m)		17.81.68 (1H, m)/
2.01 (1H, m)		20.31.63 (1H, m)/
1.94 (1H, m) 		20.1
121.62 (1H, m)/
1.40 (1H, m)		34.41.84 (1H, m)/
1.49 (1H, m)		35.61.50 (1H, dt, 13.1, 9.3)/
1.92 (1H, m)		36.11.57 (1H, m)/
1.87 (1H, m) 		37.3
13-46-47.7-47.3-47.3
14-160.2-162-161.8-159.5
155.32 (1H, m)119.15.46 (1H, dd, 3.5, 1.5)1215.45 (1H, dd, 3.8, 1.5)120.75.45 (1H, dd, 3.7, 1.7)121.6
162.02 (1H, m)/
2.15 (1H, m)		34.62.18 (1H, m)/
2.34 (1H, m)		35.92.18 (1H, m)/
2.29 (1H, m)		35.92.08 (1H, m)/
2.30 (1H, m) 		36.2
171.57 (1H, m)55.91.66 (1H, td, 10.4, 7.2)56.21.68 (1H, m)57.91.63 (1H, m) 58.2
180.90 (3H, s)18.41.04 (3H, s)191.10 (3H, s)19.41.12 (3H, s) 21.1
190.94 (3H, s)13.11.14 (3H, s)14.11.04 (3H, s)14.80.97 (3H, s) 15.7
201.68 (1H, m)39.51.84 (1H, m)41.71.19 (1H, m)41.81.87 (1H, m) 42.1
213.23 (1H, m)/
3.59 (1H, m)		63.93.34 (1H, m)/3.84 (1H, m)663.36 (1H, m)/
3.89 (1H, dd, 10.6, 3.6)		66.13.34 (1H, m)/
3.89 (1H, dd, 10.5, 3.6) 		66.3
221.94 (1H, m)/
1.22 (1H, m)		36.91.36 (1H, m)/
2.18 (1H, m)		38.51.37 (1H, m)/
2.18 (1H, m)		38.71.35 (1H, ddd, 14.7, 9.3, 7.3)/
2.22 (1H, dt, 14.8, 2.9)		38.8
233.49 (1H, m)72.93.66 (1H, td, 8.9,2.1)74.73.67 (1H, td, 8.9, 2.1)74.83.67 (1H, td, 9.0, 2.1)74.9
242.89 (1H, d, 8.1)78.43.09 (1H, d, 8.2)79.93.08 (1H, d, 8.2)79.93.08 (1H, m) 79.9
25-73.2-74.9-75-75
261.08 (3H, s)281.22 (3H, s)27.91.22 (3H, s)281.24 (3H, s) 23.9
271.09 (3H, s)241.24 (3H, s)24.11.24 (3H, s)23.91.22 (3H, s) 28
283.62 (1H, m)/
3.81 (1H, d, 10.9)		172.23.78 (1H, d, 14.7)/
4.01 (1H, d, 11.1)		177.63.78 (1H, d, 10.4)/
4.01 (1H, d, 10.4)		178.74.96 (1H, d, 2.4)/
4.88 (1H, d, 2.4)		116.1
29-70.7-70.9-69.41.83 (3H, s) 23.7
301.03 (3H, s)27.41.16 (3H, s)28.31.08 (3H, s)27.61.17 (3H, s) 27.5
1-OAc-169.5-171.6-172.5-172.4
1.99 (3H, s)20.72.06 (3H, s)20.92.00 (3H, s)21.22.00 (3H, s) 21.1
1′------4.30 d (7.8)100.3
2′------3.12 (1H, m) 75.4
3′------3.33 (1H, m) 78.8
4′------3.17 (1H, m) 72.3
5′------3.18 (1H, dd, 9.7, 2.4)77.6
6′------3.59 (1H, dd, 11.5, 5.9)/
3.88 (1H, dd, 11.4, 2.3)		63.5
Table 2. 1H and 13C NMR data for munropenes E–F (5–6) in CD3OD.
Table 2. 1H and 13C NMR data for munropenes E–F (5–6) in CD3OD.
Position56
δH (J in Hz)δCδH (J in Hz)δC
17.28 (1H, d, 10.2)161.37.28(1H, d, 10.2)161.3
25.77 (1H, d, 10.2)125.65.77(1H, d, 10.2)125.6
3 206.3 206.3
4 51.7 51.7
52.63 (1H, dd, 10.2, 5.2)462.63(1H, dd, 10.2, 5.4)46
62.03 (1H, m) 23.32.03 (1H, m) 23.3
74.14 (1H, t, 2.8, 2.8)78.34.14 (1H, t, 2.8, 2.8)78.4
8 44.9 44.9
92.25 (1H, m) 40.12.25 (1H, m) 40.1
10 41.1 41
111.68 (1H, m)/1.87 (1H, m) 18.71.66 (1H, m)/1.87 (1H, m) 18.7
121.68 (1H, m)/1.87 (1H, m) 37.31.66 (1H, m)/1.87 (1H, m) 37.5
13 47.6 47.6
14 158.7 159.1
155.49 (1H, dd, 3.7, 1.7)122.25.49 (1H, dd, 3.7, 1.7)122.2
162.03(1H, m)/2.25(1H, m) 36.12.07 (1H, m)/2.30 (1H, ddd, 15.1, 7.2, 3.5)36.3
171.72(1H, m) 57.51.66 (1H, m) 58.1
181.06 (3H, s) 21.21.06 (3H, s) 21.1
191.24 (3H, s) 19.71.23 (3H, s) 19.7
201.80 (1H, m) 40.91.86 (1H, m) 42
213.44 (1H, dd, 10.6, 6.8)/3.79 (1H, dd, 10.6, 6.8)65.33.35 (1H, m)/3.87 (1H, m) 66.2
221.65 (1H, m)/1.77 (1H, m) 38.21.34 (1H, m)/2.20 (1H, m) 38.8
234.09 (1H, m) 71.23.66 (1H, m) 74.9
243.17 (1H, m) 78.73.08(1H, m) 79.9
25 74.7 75
261.27 (3H, s) 27.21.22 (3H, s) 28
271.24 (3H, s) 26.51.24 (3H, s) 23.9
283.62 (1H, d, 11.3)/3.74 (1H, d, 11.3)66.23.62 (1H, m)/3.74 (1H, d, 11.4)66.2
291.25 (3H, s) 21.61.25 (3H, s) 21.6
301.21 (3H, s) 28.31.20 (3H, s) 28.2
1′4.32 (1H, d, 7.6)100.74.32 (1H, d, 7.6)100.7
2′3.08 (1H, m) 75.33.09 (1H, m) 75.3
3′3.34 (1H, m) 78.43.32 (1H, m) 74.9
4′3.15 (1H, m) 72.33.15 (1H, m) 72.3
5′3.23 (1H, ddd, 9.2, 6.4, 2.5)77.63.22 (1H, ddd, 9.2, 6.6, 2.4)77.6
6′3.35 (1H, m)/3.88 (1H, m) 63.63.58 (1H, m)/3.87 (1H, m) 63.6
Table 3. IC50 values (μM) of munropenes A–F (A-6) from M. pinnata in human tumor cell lines.
Table 3. IC50 values (μM) of munropenes A–F (A-6) from M. pinnata in human tumor cell lines.
CompoundsHCT116A549HepG2MCF7MDAMB
119.13>160>160>160>160
240.9>160>160>160>160
3>160>160>160>160>160
417.66>160>160>160>160
557.9>160>160>160>160
632.62>160>160>160>160
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Yang, X.; Liu, P.; Wei, Y.; Song, J.; Yan, X.; Jiang, X.; Li, J.; Li, X.; Li, D.; Lu, F. The Triterpenoids from Munronia pinnata and Their Anti-Proliferative Effects. Molecules 2023, 28, 6839. https://doi.org/10.3390/molecules28196839

AMA Style

Yang X, Liu P, Wei Y, Song J, Yan X, Jiang X, Li J, Li X, Li D, Lu F. The Triterpenoids from Munronia pinnata and Their Anti-Proliferative Effects. Molecules. 2023; 28(19):6839. https://doi.org/10.3390/molecules28196839

Chicago/Turabian Style

Yang, Xuerong, Peiyuan Liu, Yulu Wei, Jingru Song, Xiaojie Yan, Xiaohua Jiang, Jianxing Li, Xiangqin Li, Dianpeng Li, and Fenglai Lu. 2023. "The Triterpenoids from Munronia pinnata and Their Anti-Proliferative Effects" Molecules 28, no. 19: 6839. https://doi.org/10.3390/molecules28196839

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